1. Field of the Invention
The present invention is directed, in general, to the amplification of Faraday or Voigt rotation and, more particularly, to the amplification of Faraday or Voigt rotation by passing a light beam through a sample of material many times through use of multiple internal reflections and successive mirrored chambers that repeatedly send the light beam back through the sample.
2. Description of the Related Art
Faraday rotation is the rotation of the plane of polarization of light as it passes through a material in the presence of a magnetic field, whose field lines are aligned with the direction of propagation of the light. Faraday rotation, θ, is given by θ=VBL, where V is a characteristic of the material referred to as the Verdet constant, B is the applied magnetic field strength, and L is the length of propagation of the light through the material (i.e., the thickness of the sample material).
Faraday rotation is a useful tool for examining semiconductors, but many modern semiconductor materials are single or multiple thin films. For example, modern mercury cadmium telluride (HgCdTe) infrared detector devices use thin films a few microns or nanometers in thickness on cadmium zinc telluride (CdZnTe) substrates. Faraday rotation is linearly dependent on material thickness, so very thin film materials often yield little or no Faraday rotation signal. Magnets currently available, even superconducting magnets, cannot make up for the lack of rotation signal in micron and even nanometer thick films. To obtain a usable Faraday rotation signal, current technology requires the thickness of a sample to be at least a few tenths of a millimeter for most materials. Thus, there is a need for amplification of Faraday rotation in thin film materials to allow screening of electrical properties that is non-destructive to the test sample, does not contaminate the test sample, and is faster and more easily automated.
Faraday rotation is a non-reciprocal effect in that it is not dependent on the direction of light through the sample. For example, for a DC magnet whose field lines are constant in one direction, if the plane of polarization of the light is rotated to the right with respect to a stationary observer as the light travels through the sample in one direction, it will rotate the same amount to the right if the light is redirected back through the sample, adding to the original rotation. If the light is sent back through the sample multiple times, the rotation will be multiplied by the number of paths the light takes through the sample. Thus, loss of signal rotation in even the thinnest films could be compensated for if the light could be directed through the sample thousands of times.
For further details concerning basic properties of Faraday rotation, refer to H. Piller, “Faraday Rotation,” in Semiconductors and Semimetals, eds. R. K. Willardson and A. C. Beer (Academic Press, New York, 1972), vol. 8, Ch. 3, pp. 103–179. Also refer to B. J. Zook and C. R. Pollock, “Fiber Optic Tachometer Based on the Faraday Effect,” Applied Optics, vol. 28, no. 11, June 1989, pp. 1991–1994, which describes doubling Faraday rotation in a simple device. These references are incorporated herein by reference to the extent necessary to make and practice the present invention.